The present invention relates to a method for performing processes of heterogeneous photocatalysis, which process optimizes the exploitation of the radiations emitted by the light source and can be applied to large reaction volumes. Thanks to these features, the present method is highly interesting in particular for carrying out processes of heterogeneous photocatalysis on an industrial scale.
By now, a large number of photocatalytic processes in heterogeneous phase have been described in pertinent technical literature.
Such processes are characterized by the use as the catalyst, in aqueous media, of a semiconductor, such as, e.g., TiO.sub.2, on whose surface the substance is adsorbed, which has to be transformed.
By irradiation with photons having a sufficiently high energy, the promotion of an electron from the "valency band" to the "conduction band" of semiconductor is carried out.
In that way, a separation of charges, denominated "electron-hole pair" is accomplished, which migrates to the interface between the liquid phase and the semiconductor, where it is capable of causing redox reactions on the adsorbed compounds.
Many chemical substances exist, which when are individually taken and are submitted to such a treatment undergo transformations: e.g., CN.sup.-- and SO.sub.3.sup.2--, which are oxidated ([Bard, J. Phys. Chem. 1977-81 (1984)], organic acids, which are decarboxylated (Bard, J.A.C.S. 1978 100: 2239), hydrocarbon compounds, which undergo oxidation, atrazine, which is transformed into cyanuric acid.
Some substances in particular, under the hereinabove disclosed conditions, are completely mineralized, i.e., their skeleton of carbon atoms is completely converted into carbon dioxide. For example, aliphatic and aromatic halides, phenols and chlorophenols can be completely mineralized by means of heterogeneous photocatalysis [Pelizzetti, Serpone, Borgarello, Photocatalysis and Environment 467-497 M. Schiavello (Ed.) 1988]
Therefore, the great importance is evident, which such a type of reaction can have in the fight against the pollution of drinkwater and in general of aquatic environment.
In this regard, the decay due to the mineralization of, besides the hereinabove mentioned compounds, dichlorobenzenes, chloronitrobenzenes, polychlorobiphenyls and polychlorodioxines (Barbeni Chemosphere 15: 1913 1986) is known.
Such descriptions relate to processes accomplished on the laboratory scale, whilst their application on the industrial scale involves considerable problems as regards the equipment and the possibility of application to complex polluting mixtures.